Add "sage" colors to colors.py

A Sage user commented that one couldn't make plots in Sage "sage".  But this is a color.  http://xkcd.com/color/rgb/ recommends the following, and the discussion at https://groups.google.com/forum/#!topic/sage-devel/CnH45Ym4zSk indicates it could be useful as well as fun.
Merge pull request #2468 from kcrisman/patch-1

Add "sage" colors to colors.py